1. Technical Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and particularly to an image forming apparatus using an electrophotographic process to form an image and an image forming method.
2. Related Art
In an image forming apparatus of an electrophotographic system, there is known that characteristics of electrophotographic materials such as a toner and a photoconductor are changed by a change in the surrounding environment, such as temperature and humidity, or the usage time of the apparatus, and the density of an image to be formed is changed. As a result, for example, a halftone density of an image is changed, or a minute point or line can not be reproduced at the same size.
Then, in a recent image forming apparatus, in order to prevent the change of the halftone density or to ensure the reproducibility of the minute point or line, a picture quality adjustment mechanism is often mounted.
The mode of the picture quality adjustment mechanism includes a mode of using a method in which the picture quality is maintained by an open-loop control, a mode of using a method in which the picture quality is maintained by a closed-loop control, or a mode of using a method in which these are combined.
In the open-loop control, environmental conditions, the usage time of the apparatus and the like are monitored, and process conditions such as an exposure amount are changed according to a table provided in the image forming apparatus, so that the picture quality is maintained.
On the other hand, in the closed-loop control, an image of a specified image patch is developed on a photoconductor within a period except for the normal image forming operation period. Then, the patch density of the developed or transferred image is detected by a reflectivity sensor, a transmissivity sensor or the like provided in the vicinity of the photoconductor or a transfer-receiving member, and process conditions and the like are changed based on a detected density signal.
It is widely performed to stabilize the gradation reproducibility and the reproducibility of a thin line or a minute point by the open-lop control or the closed-loop control as stated above, and in general, the control as stated above is called “picture quality maintaining control”.
In a process of a general electrophotographic apparatus, after a photoconductive body such as a photoconductor is uniformly charged, a light with an intensity corresponding to the density of an image to be developed is irradiated to the photoconductor, and the potential on the surface of the photoconductor is attenuated by optical attenuation to form an electrostatic latent image. As means for irradiating the light to the photoconductor, that is, as exposing means, a laser diode or an LED is used.
Here, the deterioration of the reproducibility of the minute point is caused by the change of characteristics of electrophotographic materials, such as a toner and a photoconductor, due to a change of temperature/humidity, a change with the passage of time, a use history and the like, and it can not be avoided to a certain degree. Then, in the case where the reproducibility of the minute point is deteriorated, in order to improve the reproducibility, various picture quality maintaining control methods have been proposed. For example, there is a method in which the picture quality maintaining control is performed by changing the amount of light to expose the surface of the photoconductor or the pulse width of a light emission signal.
As this example, JP-A-2001-27837 discloses such a technique that there are provided charging means for charging the surface of a photoconductor, exposing means for forming an electrostatic latent image by irradiating a light corresponding to an image to be formed to the surface of the photoconductor charged by the charging means, a toner supply device to supply toner, and detection means for detecting the density of the toner moved from the toner supply device to the surface of the photoconductor, and gradation reproducibility is changed according to the density, detected by the detection means, of the toner moved to the surface of the photoconductor.
JP-A-2001-27837 discloses such a technique that a change in gradation reproducibility is performed by changing at least one of an exposure amount of light to expose a photoconductor, a charging potential, and data of a gradation table, and further, the change of the exposure amount is performed by an exposure pulse width modulation or pulse intensity modulation.
Besides, JP-A-11-194553 discloses a technique characterized in that an image forming apparatus to visualize an image by forming plural minute points includes means for outputting a test pattern having at least an intermediate density part, and means for performing a density correction for each of the minute points by reading the test pattern so that each of the minute points comes to have a regulated density. An intermediate value of the read density can be set as the regulated density, and a density difference shifted from the intermediate value of the read density is stored for each pixel, so that the density correction can be performed. The density correction is performed with a corrected light amount of a dot exposing laser for each pixel, and the change of the corrected light amount is performed with a pulse width modulation.
However, as in the above technique, when the correction of the density is performed by the control of the pulse width of the pulse width modulation, there arises a problem that the reproducibility becomes unstable due to the fluctuation of the response of a short pulse. Besides, in the case where the density is reduced due to the environment or the number of use years, there is also a case where the reproduction itself of the minute point can not be performed, and there also occurs a case where merely the adjustment of the pulse width is insufficient.